Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as alcohols to be used as fuels, and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes composed of carbohydrate polymers comprising cellulose, hemicellulose, and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars in a hydrolysate which can be fermented to produce useful products using a biocatalyst.
In addition to the metabolizable sugars that are present in hydrolyzed biomass, the hydrolysate includes undigested lignin and other biomass components that carry through to product isolation and downstream processes. These hydrolysate components, mixed with the biocatalyst and other fermentation broth components, need to be processed in addition to the main product. Particularly in the production of fuel alcohols, where production volumes are very high, net water use is important as is the use of fossil energy to produce the fuel alcohol. To minimize net water use, fermentation broth with product removed may be recycled to earlier stages in the process, or solids may be separated from this broth and the liquid stream recycled to earlier stages in the process (referred to as back-set). Also, the liquid stream may be purified by various methods prior to recycle. The solid stream, containing a large percentage of lignin, has low nutritional value as an animal feed, but may be used as a fuel which is burned to provide energy in the overall production process.
For separation of liquid and solid fractions in the whole stillage of the corn grain dry milling process for ethanol production, which uses grain and not lignocellulosic biomass as a source of fermentable sugars, centrifugation is typically used. The high speed horizontal decanter type centrifuges typically used are not efficient in removing suspended solids. WO2008076716 discloses use of anionic polymer flocculants to improve agglomeration of solids in centrate from the centrifuges, to aid in subsequent solid/liquid separation. Using this process a thin stillage with little to no suspended solids may be achieved. US20080153149 discloses centrifugation followed by treatment of the resulting liquid fraction (thin stillage) in a fungal bioreactor. This process uses the thin stillage as a substrate for production of high value fungal biomass, and reusable process water is also obtained.
There remains a need for efficient, low-cost processes for treatment of production side streams from fermentation broth that includes lignocellulosic biomass hydrolysate, particularly where large volumes of liquid must be processed, to produce a recyclable liquid stream and usable solids streams.